This invention relates to an alkali solution of colloidal silica for use according to a method of polishing inorganic silicate substrates at high pH values.
Application of colloidal suspensions for polishing advanced materials has become an exceedingly critical aspect of final part formation for the glass and microelectronics industries. Silica and alumina colloids are commonly utilized for polishing various microelectronic materials (copper, tungsten, silicon, etc.), and ceria colloids are frequently used for high quality glass surface processing.
Critical issues in generating high quality optical surfaces for silicate substrates, such as fused silica or silicate glasses, include the removal of surface and subsurface damage remnant from preliminary grinding processes, and the generation of relatively small topographical features with a surface roughness (Ra) typically below 5 xc3x85. For primarily mechanically-oriented abrasives such as zirconia and alumina, the final surface finish is dictated by the size and morphology of the abrasive grains. For chemical-mechanical polishing abrasives such as cerium and iron oxides, the formation of residual damage is hindered by the fact that the abrasive grains are heat treated to be a hardness comparable to that of the glass. For this reason, cerium oxide has been the abrasive of choice for the majority of glass polishing applications since the 1940""s.
The development of colloidal silica as a polishing abrasive is two fold. Colloidal silica has a spherical morphology and widely varied particle size (typically, about 20-50 nm in diameter), which minimizes scratches in softer materials. By mixing the colloidal silica in water for polishing materials such as aluminum and silicon, the surface of the metal hydrolyses and allows the abrasive nature of the colloidal silica to remove the reaction layer while minimizing interactions with the underlying surface.
For glass polishing, pH is most commonly adjusted to be acidic in order to prevent dissolution of the glass surface. This procedure has resulted in part from the fact that the different glasses will corrode and form reaction layers in widely varying fashions. By polishing glass at a relatively low pH, the glass surface does not corrode, but rather has the opportunity to chemo-mechanically interact with the cerium oxide abrasive and promote removal in a controlled manner.
An aspect of the present invention is an aqueous, alkali solution of colloidal silica for polishing inorganic silicate substrates, such as silicate-based glassesxe2x80x94aluminosilicates, borosilicates, titania-silicates, or corrosion resistant mixed alkali glassesxe2x80x94or fused silica. Preferably, the silica solutions are adjusted to a pH of 10 or above. The polished silicate surfaces have surface finishes consistently below about 2 xc3x85 Ra. Most preferably, the surface finish is about 1 xc3x85 Ra.
Although colloidal silica in neutral and acidic environments has been applied to various glasses with mixed results, the present invention processes highly polished surfaces of an inorganic silicate substrate by controlled polishing with an aqueous solution of colloidal silica having a buffered pH value of about 10 or greater. The method comprises, first polishing the glass to a surface finish below 10 xc3x85 using conventional abrasives, then applying colloidal silica in a second polishing step. This process improves surface quality by means of combined use of surface corrosion by the alkali solution and removal of the continually-forming, hydrated surface layer by the spherical colloidal silica. We also have found small particle size colloidal silica, of about 50 nm or less, to be preferred. In comparison to colloidal silica polishing of glass at lower pH, the solubility of the glass surface and the stability of the colloidal solution interfere and prevent significant improvements in surface finish. Critical to this finishing protocol is the need to remove surface and subsurface damage prior to the colloidal silica polishing step in order to prevent the alkali solution from etching the damaged areas. Furthermore, a soft polishing pad must be used during the colloidal-silica-polishing step to prevent damage commonly induced when hard pads contact silicate glass surfaces during colloidal abrasive polishing.
The method used with the present invention focuses on the use of colloidal silica polishing abrasive marketed for microelectronics applications. For the microelectronics field, material removal can be accelerated by using high pH ( greater than 10) solutions where the metal can dissolve under the alkali environment.
Our method for final polishing silica substrates comprises the steps of providing a silicate substrate, first polishing a surface of the substrate with an aqueous solution of at least one metal oxide abrasive to a surface roughness (Ra) ranging from about 6 xc3x85 to about 10 xc3x85; and further polishing the surface of the substrate with an alkali, aqueous solution of colloidal silica to a surface roughness (Ra) of about 5 xc3x85 or less. Preferably, the first polishing step polishes the surface of the substrate to a surface roughness (Ra) of about 8 xc3x85. Preferably, the further polishing step polishes the surface of the substrate to surface roughness Ra of about 2 xc3x85 or less.
The metal oxide abrasive is alumina, titania, zirconia, germania, silica, germania-doped silica, or ceria. Preferably, the metal oxide abrasive is cerium oxide.
Generally, the aqueous solution of colloidal silica is buffered to a pH ranging from about 8 to 18. Preferably, the aqueous solution of colloidal silica is buffered to a pH value of about 10 or greater. More preferably, the solution has a pH value ranging from about 10.5 to 13.5 or 14. The process relies predominately on surface corrosion by the alkali solution and partially on a preferred removal of microscopic peaks on the silicate substrate surface by abrasive pad interactions with the surface to promote improved overall roughness qualities through reducing peak-to-valley heights on both macroscopic and microscopic scales.
Typically, the silicate substrate is made of silica, fused silicates, or glasses thereof. Preferably, the silica substrate is fused silica. Generally, the colloidal silica has an average particle size of up to about 50 nm. Typically, the particle size ranges from 10 nm to 50 nm and preferably ranges from 20 nm to 40 nm or 45 nm. Particle size and surface area dimensions are understood to be greater than zero.
In one embodiment, the colloidal silica acts as a cleaning agent and removes any residual abrasive from previous polishing steps. For example, the colloidal silica removes any remaining cerium oxide from the first polishing step. A hard polishing pad carries out the first polishing step and a soft polishing pad carries out the further polishing step. Preferably, the hard polishing pad is a blown polyurethane and the soft polishing pad is a napped polyurethane.